1. Field of the invention:
This invention relates to a distributed feedback semiconductor laser device and/or a distributed Bragg reflection semiconductor laser device which attains laser oscillation at a wavelength ranging from 660 to 890 nm in a single longitudinal mode.
2. Description of the prior art:
Semiconductor laser devices, which are used as light sources for optical information processing systems and/or optical measurement systems utilizing optical fibers therein, are required to have operation characteristics achieving laser oscillation in a single longitudinal mode. As laser devices achieving laser oscillation in a single longitudinal mode, distributed feedback semiconductor lasers and distributed Bragg reflection semiconductor lasers which have a diffraction grating with a periodic corrugation on or near the active region are known (Nikkei Electronics Vol. 12, No. 22 pp 66-70 (1981)).
FIG. 2 shows a typical conventional distributed feedback semiconductor laser, which comprises an n-InP substrate 10, an n-InP cladding layer (buffer layer) 20, a non-doped InGaPAs active layer 30, a p-InGaPAs optical guiding layer 40, a p-InP cladding layer 50 and a p-InGaPAs cap layer 60, in sequence. A p-sided ohmic electrode 70 and an n-sided ohmic electrode 80 are disposed on the cap layer 60 and the substrate 10, respectively. A diffraction grating 401 for laser oscillation is formed on the upper surface of the optical guiding layer 40. This laser device utilizes the InGaPAs-InP system which achieves laser oscillation at a wavelength of as long as 1.3-1.55 .mu.m.
On the other hand, a semiconductor laser device attaining laser oscillation at a shorter wavelength of 890 nm or less comprises, as shown in FIG. 3, an n-GaAs substrate 100, an n-GaAlAs cladding layer 200, a non-doped GaAs or GaAlAs active layer 300, a p-GaAlAs optical guiding layer 400, a p-GaAlAs cladding layer 500 and a P-GaAs cap layer 600, in sequence. However, this laser has the GaAlAs cladding layer 500 on a diffraction grating 401 which is formed on the GaAlAs optical guiding layer 400, so that the GaAlAs alloy crystal is readily oxidized in the atmosphere in the formation process of the diffraction grating 401 to thereby form an oxide film on the GaAlAs optical guiding layer 400 which makes it difficult to grow the succeeding crystal (i.e., the GaAlAs cladding layer 500) thereon. Thus, it can be said that semiconductor lasers having a diffraction grating therein for laser oscillation at a wavelength of as short as 890 nm or less have not yet been sufficiently developed.
In order to eliminate this problem, a distributed feedback semiconductor laser device, in which an InGaPAs crystal is used as a material for the optical guiding layer instead of the GaAlAs crystal, has been proposed by the inventors in U.S. patent application Ser. No. 789,787. This distributed feedback semiconductor laser device is produced as follows: As shown in FIG. 4, on an n-GaAs substrate 110, an n-GaAlAs cladding layer 210, a nondoped GaAs active layer 310, a p-InGaPAs optical guiding layer 410, a p-GaAlAs cladding layer 510 and a p-GaAs cap layer 610 are successively formed by liquid phase epitaxy. An n-sided ohmic metal electrode 710 and a p-sided ohmic metal electrode 810 are then formed on the cap layer 610 and the substrate 110, respectively. A diffraction grating 444 for laser oscillation is formed on the upper surface of the optical guiding layer 410. According to the above-mentioned laser structure, the optical guiding layer 410 is not oxidized so that the growth of the succeeding crystal thereon can be easily achieved. However, the difference in the thermal expansion coefficient between the active layer 310 and the optical guiding layer 410 causes a lattice mismatch, etc., which allows the introduction of lattice distortion into the interface therebetween, to thereby create a large amount of nonradiative recombination current resulting in an increase in the laser oscillation threshold and deterioration in device characteristics.